SummaryA dynamic mathematical simulation model of tropical dual purpose (milk and beef) production systems was developed for the conditions of the Colombian tropical lowlands. The model predicts milk production and liveweight gains of cows and calves during one year or a single lactation. It includes the following components: growth of different pasture types; forage and supplement intake by grazing animals; traditional versus restricted suckling; calving season; daily milk yield (salable and consumed by calves), and growth of calves to weaning. No significant differences were found when simulated data were compared with experimental results obtained from the Centro de Investigaciones La Libertad, Villavicencio, Colombia (Martínez, et al., 1994: Hess et al., 1998). It is concluded that the simulation model adequately represents the production systems analysed, and that it can be used to study a variety of management and feeding strategies.